High-pressure water jet technology is one of the most advanced technologies in the world. Applications of high-pressure continuous water jets vary from mundane operations such as crude cleaning of edifices to highly sophisticated manufacturing of high-precision products. However, for many industrial applications, such as cleaning petro-chemical reactor vessels and mining of hard rocks, the technology, at present, suffers from serious drawbacks. This is because the magnitudes of pressures and powers required by continuous water jets for such applications are prohibitively high (>200 MPa and 250 kW per jet). The notion of using water jet techniques (forced cavitating or pulsed water jets) for such applications is a relatively new one. For example, extensive work conducted by Vijay has shown that forced cavitating and pulsed water jets can be very effective for cutting metals, etc. (Vijay, M. M., “Pulsed Jets: Fundamentals and Applications, Proc 5th Pacific Rim International Conference on Waterjet Technology, New Delhi, India, 1998). Similarly, when hard rocks are preweakened, the cutting rates will be higher and the operating costs will be lower because of the reduced wear rates and breakdowns of the cutter tools.
In the context of this specification, a distinction is made between the natural and forced discontinuous jets. The forced water jet concepts are referred to as “novel water jet techniques” in this specification. For example, a stream of high-speed droplets or slugs formed due to break-up of a continuous jet emerging in air can be regarded as a natural pulsed jet. Although natural discontinuous jets are simple to produce, their usefulness is limited because it is not possible to control their intensity and shape of the pulses which are directly related to their performance. In the case of forced pulsed and cavitating waterjets, on the other hand, it is possible to generate well-formed slugs or cavitating bubbles, by modulating a continuous water jet by high-frequency ultrasonic power resulting in enhanced performance (U.S. Pat. No. 7,594,614 B2; U.S. Pat. No. 8,297,540 B1 and U.S. Pat. No. 8,550,873 B2). However, the high-frequency cavitating and pulsed waterjets are not effective in massive fragmentation of hard rocks or rock-like materials, including explosives, such as used in landmines. The purpose of the novel electrodischarge technique disclosed in this application is to generate very powerful low-frequency (of the order of one or more pulses per second) pulsed waterjet with a precursor shock wave and subsequently a vaporous-cavitating waterjet.
Theoretically, the hydrodynamic phenomena accompanying electric discharges in quiescent liquids at atmospheric pressure have been known for more than a century. An electric discharge in a liquid at atmospheric pressure is known to cause the formation of a strong shock wave and a plasma bubble that could attain a maximum diameter of 10 mm in about 1 μs. The pressure in the plasma bubble can reach 2000 MPa or more depending on the power (voltage and current) of discharge. The interest in the technique for a variety of applications stems from the fact that these shock waves and the bubbles are sources of high power and the processing of materials is clean and can be controlled precisely (a definite advantage compared to explosives). Yutkin, for example, conducted a number of laboratory tests and demonstrated its usefulness in a variety of applications, ranging from metal forming to fragmentation of rocks, without commercial exploitation (Yutkin, L. A. “Electrohydraulic Effect,” Moskva 1955; English Translation by Technical Documents Liaison Office, MCLTD, WP-AFB, Ohio, USA, No. MCL-1207/1-2, October 1961). In at least one embodiment of the present invention, the electrodischarge technique is used to modulate a stream of water flowing through a nozzle, that is, a low-speed waterjet or, in a nozzle filled with quiescent water. According to Huff & McFall (Huff, C. F., and A. L. McFall, “Investigation into the Effects of an Arc Discharge on a High Velocity Liquid Jet,” Sandia Laboratory Report No. 77-1135C, USA, 1977), the arc discharge modulates the stream or quiescent water by three mechanisms: (1) the formation of an initial shock wave, (2) pulsed jet produced by the rapidly expanding plasma bubble and (3) the plasma bubble itself which eventually reverts into a cavitation vapor bubble. As these three hydrodynamic phenomena accompanying the discharge occur at different times, it is possible by a careful design of the nozzle-electrode configurations, as disclosed in this specification, to generate the shock only, the interrupted jet (produced by the rapidly expanding plasma bubble) only or, the cavitating waterjet only or, all the three phenomena in tandem to inflict immense damage on a target material. The nozzles shown in FIG. 1 and FIG. 2, for example, are meant to produce only shock waves. Since the frequency of operation is usually low (≈1.0 Hz), in the interrupted mode, the technique basically functions as a water cannon.
Generating shock waves in water by electric discharge is disclosed in U.S. Pat. No. 3,364,708 (Padberg). A shock plasma earth drill is disclosed in U.S. Pat. No. 3,679,007 (O'Hare). Various plasma blasting techniques are disclosed in U.S. Pat. No. 5,106,164 (Kitzinger et al.), U.S. Pat. No. 5,482,357 (Wint et al.), U.S. Pat. No. 6,283,555 (Arai et al.), U.S. Pat. No. 6,455,808 (Chung et al.), U.S. Pat. No. 6,457,778 (Chung et al.), and U.S. Pat. No. 7,270,195 (MacGregor et al.). In the foregoing patents, a probe with electrodes (e.g. coaxial electrodes) is inserted into a borehole in the rock formation which is then filled with water or electrolyte.
Although the prior art provides a qualitative description of the phenomena accompanying the electrical discharge in quiescent water, there is scant information with respect to the discharge in a moving stream of water. Therefore, the inventor has conducted extensive semi-theoretical (computational fluid dynamic analysis) and experimental work on the electrodischarge technique for the conceptual nozzles shown in FIG. 1 and FIG. 2. FIG. 3, for example, shows the very high pressures generated by the impact of a shockwave on the target material (Vijay, et al., “Modeling of Flow Modulation following the electrical discharge in a Nozzle,” Proceedings of the 10th American Waterjet Conference, August 1999). The flow rate through the nozzle was 13 usgal/min at a pressure of 5 kpsi in the vicinity of the electrodes. The orifice (nozzle) diameter was 0.085 in. The magnitude of the electrical energy dumped between the electrodes was 20 kJ and the shock impact was at 81.2:s after the discharge. FIG. 4 shows the effect of placing a reflector upstream of the electrodes (the tip of the central electrode (d∀) in FIG. 1 (shown clearly by #29 and #29a in FIG. 11). The target is placed at 5 in from the nozzle exit. It is seen that at a time (t) of about 30:s, the plasma expands sending a shockwave S1 towards the nozzle exit and a shockwave S2 towards the inlet. Shockwave S1 leaves the nozzle at approximately 50:s and forms a high-speed wave (W1) which accelerates the front F1 of the original steady jet to F2. The front F2 impacts on the target at 78.2:s producing a peak pressure of 2,600 MPa at 81.2:s as shown in FIG. 3. Shockwave S2, on the other hand, is reflected as shockwave S3. This shockwave on passing through the plasma emerges as shockwave S4 and ultimately causes another high-speed wave W2 in the jet impacting the target at 104:s, creating pressure peaks, of the order of 1,700 MPa. These semi-theoretical results show the advantage of using a reflector in the nozzle configuration.
As illustrated in FIG. 5A, further computational fluid dynamic analysis has indicated the occurrence of multiple peaks in the impact pressure. This is due to the fact that the discharge voltage, as illustrated in FIG. 5B, is a decaying sinusoidal wave (Yan, et al., “Application of ultra-powerful pulsed Waterjet generated by electrodischarges,” Proceedings of the 16th International Conference on Water Jetting, France, October 2002). Thus, by proper design of the discharge circuit, it is possible to generate multiple shockwaves to impact the target, enhancing the performance of the pulsed waterjet generated by the electrodischarge technique.
The phenomena accompanying the discharge depend on several operating variables and configurational parameters of the electrode-nozzle assembly. The operating variables are the pressure in the chamber, which could be of the order of 15 kpsi (could be any pressure although a range of 10-20 kpsi provides good results), flow (determined by the orifice diameter, do, of the orifice, typically of the order of 13 usgal/min although a flow of 10-15 usgal/min provides good results), or quiescent water (depends of the volume of the nozzle chamber, typically of the order of a liter), magnitude the voltage (V) of the capacitor (typically of the order of 20 kV, but could be as high as 100 kV), capacitance (C) of the capacitor, energy (Ec) stored in the capacitor (Ec=0.5CV2). Depending on the capacitance, the energy stored in the capacitor bank could be as high as 200 kJ. Although the energy of discharge can be varied either by varying the voltage or the capacitance, to keep the size of the system compact, it is better to vary the voltage and the duration of discharge (θ), which will depend on the magnitudes of L-C-R (inductance, capacitance and resistance) of the discharge circuit.
As indicated in FIG. 1 and FIG. 2, the configurational parameters are: the shape (contour) of the nozzle chamber to focus and propagate the shockwaves towards the nozzle exit, the shape (conceptual designs are illustrated in FIG. 7 and FIG. 8), diameter (d∀), location (k) of the electrodes from the nozzle exit, the gap (ι) between the electrodes. For example, as shown conceptually in FIG. 7, the inner contour of the nozzle could be an exponential curve and, in order to obtain smooth flow of water, the outer profile of the electrode would also be exponential, providing generally parallel surfaces.
As further illustrated in FIG. 1 and FIG. 2 and also, in the conceptual configurations shown in FIG. 7 and FIG. 8, there are several different shapes, size and dispositions of the electrodes in the nozzle. These figures also show two possible configurations of the electrodes. Whereas the purpose of the short plasma channel (FIG. 1) is to generate cavitation bubbles in the stream, that of the long channel is to produce a high-speed pulsed water jet (Vijay and Makomaski, “Numerical analysis of pulsed jet formation by electric discharges in a nozzle,” Proceedings of the 14th International Conference on Jetting Technology, 1998). From the standpoint of performance, the most important geometric parameters are (as shown in FIG. 1 and FIG. 2) the magnitudes of D/do, the distance k, the distance (gap) between the electrodes ι, the inner profile of the nozzle and the shape and disposition of the electrodes. These geometric parameters also determine the operating parameters such as the pressure of the liquid, electrical energy and frequency, etc. As an example, test results are illustrated in the plot of FIG. 6. For the given set of operating parameters listed in the legend, the speed of the pulsed waterjet depends considerably on the gap (ι) between the electrodes. The data clearly show that it is possible to increase the speed of the jet by almost a factor of three by simply increasing the gap between the electrodes from 6 to 22 mm. This method affords a simple means to significantly increase the speed of water slug without increasing the input electrical energy. This is very important for many practical applications such as neutralization of landmines where a pulse having a very high speed (≈1000 m/s) is required.